The prior art reveals various methods for decreasing the distortion which results during the precipitation hardening of formed parts produced from copper beryllium alloys. Unfortunately, these prior art methods are minimally effective and often fail to control the resultant distortion to a commercially acceptable degree. Additionally, the prior art methods yield inconsistent non-reproducible results. These alloys are used in electrical connectors where consistent dimensional and mechanical properties in the finished product are important.
Basically, all prior art methods for producing formed parts from copper beryllium alloys include the combination of the following sequence of processing events: preparing a copper beryllium melt; casting the melt; hot working the cast copper beryllium; solution annealing the copper beryllium; cold working the solution annealed copper beryllium; forming the copper beryllium; and age-hardening the formed copper beryllium. As mentioned above, various methods have been developed in an attempt to control the distortion experienced in this processing sequence.
In this connection, reference is made to the methods disclosed in Goldstein U.S. Pat. No. 4,425,168, McClelland U.S. Pat. No. 4,394,185, Wickle U.S. Pat. No. 4,179,314, Shapiro U.S. Pat. No. 3,882,712, Britton U.S. Pat No. 3,658,601, the article entitled "Residual Stresses in Copper-2% Beryllium Alloy Strips", authored by K. E. Amin and S. Ganesh, Experimental Mechanics, December 1981, page 474, and the article entitled "A Technique For Predicting Distortion And Evaluating Stress Relief In Metal Forming Operations", authored by K. E. Amin and R. M. Rusnak, Journal of Metals, February 1981.
The methods disclosed in these prior art sources are only partially successful in eliminating distortions in finished products. Amin and Ganesh have correctly identified residual stresses as one of the sources of distortion. Amin and Ganesh have also shown that a high rolling reduction of copper beryllium strip results in tensile residual stresses near the surface of the strip and compressive residual stresses at the center of the strip while low rolling reductions result in the opposite location of these stresses within the strip.
The McClelland and the Britton patents comprehend the importance of relieving residual stresses prior to the forming operation by the incorporation of a pre-aging technique. However, they fail to realize that in a thermal treatment such as their pre-aging technique two reactions occur simultaneously. On the one hand thermal treatments such as pre-aging promote the nucleation and growth of the precipitates formed during precipitation hardening. On the other hand these treatments also reduce the magnitude of the existing cold working and residual stress patterns that affect the precipitation hardening. The recognition of these competing mechanisms is critical in the development of reproducible softening and hardening techniques and the effects thereof on the reproducibility of the formed parts. All thermal treatments must utilize those combinations of times and temperature which relieve or decrease the magnitude of residual stresses before the formation of precipitates become dominant.
None of the prior art teachings recognize that the rates at which the nucleation and growth of precipitates occur are different when the metal matrix is aged under tensile residual stresses as opposed to compressive residual stresses. The realization of the existence of these differential zones of nucleation and growth is critical in the development of a process for producing distortion free copper beryllium products since consistent and reproducible mechanical and dimensional properties can only be obtained from the aging process in coils of strip, wire and the like, and parts formed therefrom by combinations of thermal and mechanical treatments that yield consistent and reproducible stress and precipitate patterns. The possible existence of these patterns of undissolved precipitates or precipitate nuclei has not heretofore been considered.
Furthermore, to date there has been no application of the effects of light and heavy reductions to the controlling and leveling out of residual stresses within the alloy or to the modification of precipitate patterns left after an incomplete solution anneal.